Absorption column design packed, mass transfer

Density and viscosity are important for the process and plant design, especially relating to fluid dynamics calculations for the absorption column, gas-liquid mass transfer resistance and the selection of pumps and packings. The measurement systems were first validated with 30 w1% MEA solutions. As seen in Figure 5, the dynamic viscosity of MEA solutions is significantly increased by the dissolution of CO2 and the measured data agree well with the values predicted from Weiland s correlation [20]. The maximum deviation is less than 3%. 

To determine the required size of an absorption or stripping unit, it is necessaiy to know not only the equilibrium solubility of the solute in the solvent and the material balance around the column but also the rate at which solute is transferred from one phase to the other within the tower. This rate directly affects the volume of packing needed in a packed tower, the degree of dispersion required in a spray contactor, and (somewhat less directly) the number of trays required in a tray tower. The last effect occurs as a result of the influence of mass transfer rate on tray efficiency which is discussed in a later section. Because of its direct effect on packed tower design and the importance of this type of contactor in absorption, this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theory is given in Chapter 2. 

In 1966, in a paper that now is considered a classic, Danckwerts and Gillham [Tmns. Inst. Chem. Eng., 44, T42 (1966)] showed that data taken in a small stirred-ceU laboratoiy apparatus could be used in the design of a packed-tower absorber when chemical reactions are involved. They showed that if the packed-tower mass-transfer coefficient in the absence of reaction (/cf) can be reproduced in the laboratory unit, then the rate of absorption in the l oratoiy apparatus will respond to chemical reactions in the same way as in the packed column even though the means of agitating the hquid in the two systems might be quite different. 

Though packed absorption and stripping columns can also be designed as staged process, it is usually more convenient to use the integrated form of the differential equations set up by considering the rates of mass transfer at a point in the column. The derivation of these equations is given in Volume 2, Chapter 12. 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

Packed columns have been used in chemical industry for more than a century. The packing in a packed column provides a large surface area over which the gas contacts the liquid and the gas-to-liquid (and vice versa) mass transfer occurs (Fig. 3). As a result, the gas-liquid mass transfer process in a packed column is very efficient. Research and development on packed column has received much attention. One of the earlier and original literatures reported by Leva covers the principles of gas-liquid mass transfer in a packed bed, as well as packed column design and column internals. Recent reported literatures on gas absorption in a packed column from Fair et al., Strigle, and Billet are excellent references on transport phenomena in a packed column and packed column design. 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

The packed column is now a widely used device for mass transfer in distillation, absorption, and stripping. Typical of the packings which are available today are those shown in Figs. 13-4 and 13-5. Recently, Bolles and Fair3 presented the results of an excellent evaluation of the design equations for packed columns. The most reliable equations or correlations for making the following types of determinations... 

Packed columns are often used for distillation, liquid-liquid extraction, and humidification as well as for gas absorption. The design can be based on overall transfer coefficients or on the number of transfer units and the height of a transfer unit. For distillation or humidification, where the gas phase is continuous and the liquid flows in rivulets over the packing, the mass-transfer coefficients and flooding characteristics are similar to those for gas absorption, and the same generalized correlations would apply. 

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